1. Field of the Invention
The present invention relates to an antenna drive apparatus for use in orientation-controlling an antenna, such as an antenna system for optical communication, in a desired direction.
2. Description of the Related Art
Usually, a stepping motor has been used as a drive source in an antenna drive apparatus and, by the stepping motor, the antenna can be orientation-controlled with high accuracy. In the stepping-motor it is possible to compute a drive angle (full-step angle) from a product of a step angle and drive pulse number as shown in FIG. 4. By doing so, the stepping motor can have its rotation stopped to a predetermined position without being magnetically excited, that is, through the utilization of the so-called detent torque, and be held to a "power OFF" position corresponding to a non-excited state, thus ensuring accurate reproducibility.
With the antenna drive apparatus, on the other hand, the stepping motor is intermittently driven by drive pulses corresponding to those into which a drive current is converted (see FIG. 4), that is by the so-called full-step drive pulses, to enable the antenna to be orientation-controlled. However, at each drive of the motor, great vibration is exerted on the antenna due to a disturbance caused by the motor drive, thus exerting a bad influence on stable orientation control.
In the antenna drive apparatus, the stepping motor is driven by microstep drive pulses corresponding to those into which the step angle is divided as shown in FIG. 5. By doing so, the so-called microstep drive system is adopted to achieve a smooth motor drive. The use of this system can reduce vibration caused in the antenna due to disturbance upon the driving of the motor. The microstep drive pulses are generated as pulses to which a drive current is converted as a pulse equivalent.
The above-mentioned microstep drive system, however, cannot utilize a detent torque at each step and, when the stepping motor is turned OFF, then the stepping motor is sometimes stopped to a position not just corresponding to the step angle involved (a position as indicated by a broken line in FIG. 4), thus failing to achieve its reproducibility.
In this case, any accurate control is difficult to achieve unless a position control system is provided so as to ensure the continuous supply of electric current at all times without cutting off the power source. This system involves more dissipation power. Further it is also necessary to consider the control of heat generated during operation. Cumbersome handling is required in this respect.